1. Field of the Invention
The present invention relates to a polyphonic musical instrument wherein tones are produced by computing a master data set, transferring the data to buffer memories, and converting buffer memory contents to musical sounds.
2. Description of the Prior Art
The advantages of digital waveshape generation in an electronic musical instrument are outlined in U.S. Pat. No. 3,515,792 and U.S. Pat. No. 3,809,786. Such advantages include
A. realistic simulation of organ tones and other musical sounds such as piano, flute, bells, plucked strings; PA1 B. production of the same waveshape, and hence tonal quality, regardless of which note or octave is being played; PA1 C. simplified implementation of both foundation and mutation stops; PA1 D. controlled selection of the attack and release characteristics of the produced musical notes; PA1 E. all electronic operation; and PA1 F. ease of construction using batch fabricated, digital microelectronic techniques.
In the organ described in U.S. Pat. No. 3,515,792, musical notes are produced by storing a digital representation of a waveshape characteristic, e.g. of an organ pipe tone, and repetitively reading out this stored waveshape at a selectable clock rate determining the fundamental frequency of the produced note. Stored in the waveshape memory are the actual amplitude values at a plurality of sample points. A frequency synthesizer produces a clock signal at a rate determined by the note selected on the organ keyboard or pedals. The stored amplitudes or amplitude increments are read out of the memory repetitively at the selected clock rate (which differs for each note) to generate the selected musical tone. Attack and decay is provided by programmed division, or division and subtraction, of the read out amplitude or increment values.
In the organ described in U.S. Pat. No. 3,809,786, musical notes are produced by computing the amplitudes at successive sample points of a complex waveshape and converting these amplitudes to notes as the computations are carried out. A discrete Fourier algorithm is implemented to compute each amplitude from a stored set of harmonic coefficients C.sub.n and a selected frequency number R, generally a non-integer, establishing the waveshape period. The computations, preferably digital, occur at regular time intervals t independent of the waveshape period. At each interval t the number R is added to the contents of a harmonic interval adder to specify the waveshape sample point qR, where q = 1,2,3, . . . For each point qR, W individual harmonic component values C.sub.n sin(.pi.nqR/W) are calculated, where n = 1,2,3, . . . ,W. These values are algebraically summed to obtain the instantaneous waveshape amplitude, which is supplied to a digital-to-analog converter and a sound system for reproduction of the generated musical note. Attack, decay and other note amplitude modulation effects are obtained by programmatically scaling the harmonic coefficients. In a polyphonic musical instrument system, time sharing and multiplexing is used to calculate separately the sample point amplitudes for each selected note, these amplitudes being combined by summation to produce the desired ensemble of musical sound.
The DIGITAL ORGAN described in U.S. Pat. No. 3,515,792 is not readily adaptable to modern musical instruments of the synthesizer variety wherein the tonal characteristics of a note must be capable of smooth continuous time variations. The waveshape stored in memory is a rigid representation of a prespecified tonal structure. Expensive digital filters are required to modify the harmonic structure of the stored waveshapes. Another serious drawback inherent in the use of stored waveshapes is the need for high system logic clock frequencies in a time-shared implementation of a polyphonic system. Tone synthesizers require tones corresponding to about 32 harmonics. At C.sub.7, the 32'nd harmonic yields a frequency of 2093.times.32=67Khz; far above the audible range. The effective single channel clock frequency required to read such a waveshape at C.sub.7 is 2.times.67=134Khz. A time shared 12 note polyphonic system that operates by multiplexing a single waveshape memory would require a minimum system logic clock of 1.6Mhz.
The COMPUTER ORGAN described in U.S. Pat. No. 3,809,786 overcomes many of the modern tonal musical problems caused by the inflexible waveshape in memory characteristics of the Digital Organ. The Computer Organ has a very severe requirement for fast system logic clocks. For a single channel generating a 32nd harmonic tone at C.sub.7, the system logic clock must operate at a frequency of 4.29Mhz. A timed shared 12 note polyphonic system using a single computation channel requires a minimum system logic clock of 51.43Mhz. If harmonic limiting is used with the Computor Organ as described in U.S. Pat. No. 3,809,789, then for a maximum frequency of 20.9Khz (tenth harmonic of C.sub.7), a single channel system requires a clock at 1.34Mhz and a 12 note polyphonic system requires a minimum system logic clock of 16.1Mhz. Further reduction of the system clock frequency can be accomplished by using additional circuitry as described in U.S. Pat. No. 3,809,788.
An object of the present invention is to provide a polyphonic electronic musical instrument wherein time varying waveshape synthesis is accomplished in a manner totally different from that known in the prior art, yet exhibiting all the above listed advantages of digital waveshape generation while using clock speeds compatible with economical batch fabricated digital microelectronic devices.
Other objects and features of the invention will become apparent in conjunction with the following descriptions and drawings.